I've finished building my ChangFa generator project and had it producing
I borrowed a friends "Kill-A-Watt" meter to monitor the output voltage and
freq of my genset. I adjusted the engine speed to indicate 60hz and the
voltage was at 127V. I used the meter to measure the house voltage and it
measured 120V, with power provided by the power company.
The ST-10 style genhead can have the output voltage adjusted by
installing a 5 ohm, 200W rheostat. Where does one find such a beast?
Or is 127V an acceptable voltage?
If you were using the generator long term you would
probably notice short life on incandescent lamps, but I
doubt any other common household stuff would have a
Unless you can get a surplus rheostat cheap you might want
to wait til you get a typical load on the generator and
then check the voltage. A rheostat that large will cost
close to $100. A variable resistor would be cheaper, but
not as convenient since you wouldn't be able to adjust it
on the fly.
1. If you put a load on the generator, the voltage will drop.
2. 127 volts is a "little" on the high side, but acceptable.
3. If you need to adjust the voltage, a Variac is a far better choice than
a rheostat. A Variac is an autotransformer with an adjustable tap. You can
adjust the voltage without throwing away any power. Further, it can raise
as well as lower the voltage. Under full load, the generator will probably
output a voltage that is a little on the low side... You can find one at an
electronics surplus store. They're not cheap, at least not for a big one,
but they work very well.
I assume the rheostat goes in the field circuit, not on the
generator's output, so a variac would not be appropriate. A
5 ohm/200w resistor would not come close to handling the
High voltage for 120V circuits is actually 135V so you are well within the
I'll also note that it is always nice to have that higher voltage as motors
will put out their torque a lot nicer and you also have some headroom for
long extension cords that are usually never big enough for the currents that
they are carrying.
Why isn't there an Ozone Hole at the NORTH Pole?
The frequency is generally no more and often a lot less important than the
voltage. Power companies maintain a highly accurate frequency for
timekeeping and even raise or lower it slightly to keep the clocks on time.
The generator frequency will not be accurate enough for time keeping unless
it has a very sophisticated governor. For emergency use it is general
practice to set the no-load voltage and frequency on the high side and let
the governor and voltage regulator do their best at maintaing a satisfactory
output as the load varies.
The highly accurate frequency requiement is not for timing, it is to
allow the national electical grid (acutally the east and west coast
grids) to function. When you have hundreds (thousands?) of multi
megaawatt generators all essentially hooked up parallel, you have to
have the frequency variation set as small as technically possible. If
not, you get some massive eddy currents in the system.
D> The frequency is generally no more and often a lot less important than the
The phase of the frequency determines where the power goes on the grid.
Lead the grid and you pump power into the grid while matching the grid phase
and you won't pump anything into it. A really neat trick.
Why isn't there an Ozone Hole at the NORTH Pole?
Let's say 2 gens in a grid with a phase difference of 180deg:
Who's leading, what's the voltage on the grid and where is all the power
going that is feeded in?
You can make that mind game with varying phase differences, but the
result is not so obvious.
Each generator will look like a double direct short circuit to the
other. I use the term "double short circuit" to mean a 240 volt
generator trying to feed not a short circuit, but a negative 240 volt
short circuit. Current wil be limited by the sum of the internal
resistances of the generators. For a few seconds anyway.
Nick Müller wrote:
Yes, kind of. Or better the bunny and rabbit story.
Trying to solve the following situation:
If a gen has to be a tad ahead in phase*) to feed power into a grid, and
there are 3 gens that want to feed, which one has to be ahead?
*) as Bob says
Think of it this way: Suppose your generator is hooked to a network that
has lots of generators and lots of power users (motors, TV's, and
electric ovens). In this case, nothing your generator can do will make
any difference to the grid. If your generator is not running and you
hook on, it will behave like a motor, try to spin the engine on your
generator set. Asssuming it has sufficient torque (not likely), it will
spin up to synchonous speed (1800 or 3600 rpm in the US) or perhaps to a
bit less to allow "slipage" in the typical induction motor winding. Your
watt meter will spin in the useage direction.
Now fire up the engine and spin it up to the synchonous speed. The
generator will match the grid voltage and pahse angle. No current flows,
you watt meter stops.
Now try to increase the generator speed. As soon as you try, the
generator phase angle will allow current to flow into the lagging phase
angle of the grid. Exactly how far you can advance it is a function of
the inherent resistance in your generator and the lines to the grid as
well as the horsepower of the engine. Since the load is essentially
infinite and the engine isn't, your engine will run at sychronous speed,
phase angle leading the grid by a tiny amount. Put a watt meter on the
line and it will show watts roughly equal to the shaft hp of your engine
(minus the usual effiency losses)
Nick Müller wrote:
Here you are (kind of) right. But the wattmeter shows your short circuit
losses with what you try to "fight" the net. And these losses are 100%.
To see your error, draw a sine wave on some paper. Copy that sine onto a
transparent paper. Lay the transparent sine wave over the other, so that
Important: Now, I'm just talking about the positive half wave. Look at
the sine avove the phase axis.
Now shift either one a bit (10 deg, to make it clearer) to the left (or
right). You have a lag/lead in the phase. No take a _close_ look on
what's happening. On the raising slope, you have a positive difference
in voltage, in the falling slope you have a negative difference. Where's
your power going now? On the raising slope you feed, in the falling
slope you sink.
Right, in the drain.
To make a contribution to the net, you need a postive voltage difference
all the time. You get this, when you are exactly in pase and have a
higher voltage than the net.
read as an overexaggeration
of your "out of pahse theory"
Before you start nitpicking about "higher and lower voltage", replace it
with sinking and sourcing current, if it helps.
This proposed concept makes absolutely no sense to me. Alternating current
same as battery charging, which is sorta what is implied by "leading the
phase" and "pumping power into".
Leading the phase by 5%, for example, also means that it's lagging by 95%.
Differences in potential that will be present, will have the effect of
The way more power is available is that the output current capacity of the
"grid" is increased by adding more parallel precisely-in-phase current
I would think that it's highly likely that special equipment is used to
synchronize the phase and frequency of a generator's output before the
A mismatch in phase or frequency should be avoided, not intentionally
induced. Mismatches will cause dramatic currents due to differences in
The out-of-phase reaction can be observed when parallel connecting the
secondary windings of a dual-secondary power transformer. The out-of-phase
current is maximum opposed currents.
This test is best done with a test lamp to avoid destruction of the
When the dual secondary windings are properly phased, the current capacity
For a transformer with dual 12V 5A secondaries, the output current capacity
would be 10A.
This same practice is followed when connecting secondary windings of
One would not attempt to parallel the output terminals of an individual
3-phase transformer for the same reasons. The out-of-phase currents would
cause instant destruction of the transformer.
There isn't any practical reason to try to determine which phase leads, it's
not relevent and has no meaning in AC power usage. In your home L1 and L2
are opposed, and that's not an indicator of which one occurs first in real
The same with 3-phase, it's just the separation of the phases that gives it
In working with either one in home or shop environments, load balancing is
about the only important consideration, with safety being at the top of the
I remembergong to a power station in Lancashire. They had a meter,
presumably connected to the three phases of the mains grid and also to the
three phases of the local generator. On the face of this meter was shown
"Lead," "0," and "Lag."
When they had to run the local generator, they controlled its speed until
the meter stopped rotating and only when it settled down at the "0" position
would they operate the circuit-breaker to connect to the grid. As you say,
if there were a severe error. it would result in breakers tripping out and
I don't know if this has any relevance to your problem, but I thought I
might contribute it, anyawy.